1. Field of the Invention
The present invention relates generally to a recording media certification process, apparatus and article of manufacture and, in preferred embodiments, to such a process, apparatus and article for testing magnetic recording discs, using a wide write head and, preferably, for use with a skip track testing procedure.
2. Related Art
During the manufacture of computer data recording media, such as magnetic recording discs, flaws may be inadvertently formed in the recording surface. Such flaws may include, for example, tiny pinhole-like discontinuities, thinning or other anomalies in the magnetic coating that forms the recording surface. These areas of a disk may not hold a magnetic field pattern reliably and, thus, may be unsuitable for storing data.
In this regard, magnetic recording discs which are used in magnetic disc drives are typically screened, or tested for magnetic defects during or after manufacture. Generally, such certification processes involve electromagnetically writing a signal on the disc and then reading the recorded magnetic signal back. The write signal may be, for example, a standard amplitude sine wave signal, or other suitable testing signal. The signal read back from the disk is then evaluated and, based on certain signal characteristics or quality criteria, a determination is made as to whether or not the disc functioned properly to record the written signal. For example, weak read signals can indicate a defect in the recording surface of the type noted above.
Therefore, a track average amplitude of the signal read back during the testing procedure may be analyzed for defects. If the amplitude drops below a predetermined threshold level, a defect is identified and recorded. The number and length of these amplitude drop-outs are used to certify the quality and acceptability of the disc.
The quality of the recording surface of a disc may be tested in this manner, for example, by writing and reading test tracks on the entire recording surface. However, for purposes of minimizing testing time and maximizing the output of a given tester, procedures are typically implemented for testing only a portion of the disc recording surface. Based on the tested portion, inferences are made regarding the quality of the remaining portion of the recording surface. Thus, were only a fraction of the disc is tested and the total number of defects identified during the test is factored by the area tested versus the total area of the disc.
Two common types of such test procedures are referred to as "spiral testing" and "skip track testing." According to spiral testing procedures, separate read and write heads are mounted on separate linear actuators. The write head is controlled to continuously write a track on the disc and the read head is controlled to read back the signal, as discussed above. However, both the read head and the write head are continuously moved from the outside of the disc to the center of the disc, as the disc is rotatably driven on a spindle, to define a spiral path of motion relative to the disc surface. The ratio of the spindle rotation speed and the linear actuator speed determines the pitch of the spiral. In this manner, the tested portion of the disc comprises a spiral test track extending between the outer peripheral edge of the disc surface and the center hub of the disc.
With skip track testing, a single magnetoresistive (MR) head having both read and write transducers may be used. According to typical skip track testing procedures, the head is controlled such that, during one disc revolution, the write element writes a signal on one track of the disc. On the next revolution the read element reads back the signal recorded during the previous revolution. After the signal is read the head is stepped to the next track location of interest, typically skipping one to two track widths for purposes of minimizing testing time and maximizing tester output, as noted above.
This process may be summarized as follows:
1. Write a track around the circumference of the disc in one complete revolution.
2. Read the previously written track in the next complete revolution.
3. Step to the next track (skipping one or two track widths).
4. Repeat the sequence.
If no tracks are skipped in step 3, then the minimum time required to test a given disk is as follows:
testing time=[(tested width).div.(read track width)].times.(time for one revolution).times.2 PA1 defect size=read track width * (1 - Threshold level),
where the "tested width" is equal to the total width of the recording surface (in the radial dimension of the disk) minus the total width of all skipped track widths; and where the "read track width" is the width (in the radial dimension of the disk) of the annular path on the recording surface traced by the read transducer during a reading operation.
Thus, the minimum testing time may be reduced by skipping track widths in step 3 to reduce the "tested width". However, defects in the recording surface locations that were not tested (e.g., the skipped track widths) are not detectable with standard skip track processes. Accordingly, a disc may pass a standard skip track testing procedure, yet include significant undetected defects. On the other hand, attempts to improve the testing accuracy of standard skip track testing processes by eliminating or reducing the number of the skipped track widths, result in increased testing time and, thus, decreased testing efficiency.
Heads which have been used for conventional certification processes have typically been the same type of heads which are designed and used in disc drive products. A generalized representation of a typical MR head configuration 10, having a read element 12 and a write element 14 is shown in FIG. 1. A mid shield 16 and a lower shield 18 are typically provided to shield the read element from spurious electromagnetic energy.
During operation, the head 10 is disposed adjacent a disc recording surface such that, upon rotation of the disc, the head, in effect, sweeps over a portion of the recording surface. For each revolution of the disc, the write element traces out a circular ring on the recording surface of the disc, referred to as the write track or, in disc certification processes, a test track. The read element also traces out a circular ring on the recording surface. The ring traced by the read element is aligned so as to be coincident with a write (or test) track during a read operation. In conventional heads, the read element is typically dimensioned slightly smaller in width than the write element width, such that the ring traced by the read element may be aligned to be fully within a previously recorded write track during a read operation. However, conventional read elements are typically wide enough to cover almost the entire width of a write track, so that data written on the track is not missed by the read element's traced ring.
Disc drives typically use a rotary actuator to step the head across the surface of the disc. To accommodate the rotary actuator the write element may be offset relative to the read element, as shown in FIG. 1 as offset distance D. However, conventional disc certification processes typically use a linear actuator, such that the above-noted offset can cause the read element to be misaligned and/or incompletely cover a test track recorded by the write element. Accordingly, in such conventional processes, minor adjustments to the skew of the certification head are typically necessary, to insure that the read element is properly aligned with respect to the write element (and the write or test track).
To improve the efficiency of disk testing processes, disk certification heads have been proposed with widened read and write elements, for defining widened test tracks. However, as the read element width is increased the sensitivity of the head to small defects tends to be reduced. Typically, defect sizes as small as one micron must be detected during a disk certification procedure. A generalized model for determining the maximum read track width necessary to detect a given defect size may be expressed as follows:
where the "Threshold level" refers to a selected percentage of the track average amplitude. The maximum usable threshold level is generally, for example, 85%. For a defect size of one micron, the above model leads to a maximum read track width of five to six microns.
To increase the aerial density on the discs today, a modem trend is to reduce the track widths. This leads to smaller acceptable defects, which in turn leads to narrower certification heads. However, as the read track width of the certification heads is reduced the certification time tends to increase correspondingly. Accordingly, there is a need in the industry for an improved disk certification process, apparatus and article of manufacture which avoids the above-noted error size detection problems associated with increasing the widths of the read and write elements of a head, and which avoids the efficiency problems associated with reducing the widths of these elements.